Sludge dewatering system

ABSTRACT

A system for the rapid dewatering of sludge in large quantities makes use of a filter plate of design and construction capable of supporting heavy mechanized mobile equipment and without damage to the filter media. The filter plate features a monolithic plate of granular filter material used as the filtering medium in sludge beds for dewatering or reducing residual solids or sludge of potable water treatment systems. The filter material is made structurally rugged and with a smooth hard surface for the retention of sludge solids. The filter material is supported upon a substructure of aggregate providing approximately 40% internal voids for the gravity flow or drainage of filtrate and the back flow of chemicals and gases used in the rapid water reduction process. The assemblage is constructed of such strength as to allow for use of heavy handling equipment such as front end loaders in the removal of the dried sludge or cake upon the completion of the rapid water reduction process. The process follows a cyclical pattern on adjacent sludge beds where raw sewage is pretreated with a polymer coagulant to break up solids to flocculate the sludge particles and spread on the filter bed as the filter bed is being filled. Large coagulated sludge particles gain weight as they settle and water is drawn off by gravity while a bed is being filled to the desired designed level. Smaller sludge particles pass to the voids below by a vacuum of 10 to 15 inches when the vacuum is applied, to start the rapid dewatering process through the larger flocculated sludge particles and the mass may be periodically moved and heated while dewatering progresses. Upon completion of dewatering, one or more units of front end loading equipment is used to move the dried sludge cake into heaps and load it into trucks for transport to disposal sites.

BACKGROUND OF THE INVENTION

This is a continuation of copending application Ser. No. 202,241, filedOct. 30, 1980, now U.S. Pat. No. 4,382,863 which is in turn acontinuation of copending application Ser. No. 098,887, filed Nov. 30,1979, now abandoned, which was in turn a continuation of copendingapplication Ser. No. 930,529, filed Aug. 2, 1978, now abandoned, whichwas in turn, a continuation-in-part of copending application Ser. No.739,602 filed Nov. 8, 1976, now abandoned.

FIELD OF THE INVENTION

This invention relates to a sewage or water treatment system. Theinvention is directed particularly to improvements in rapid sludgedewatering or sludge reduction beds associated with such systems, andprogressive removal of sludge cake as a continuing operation bymechanical means.

BACKGROUND OF THE INVENTION

In the past, technological advances in the treatment of sewage have beendirected for the most part to the various processes utilized in thereduction of raw sewage, such as aeration, settling coagulation,chemical precipitation of metallic ions, etc. The end products of allsuch sewage from water treatment facilities, however, are a cleareffluent and waste sludge. This invention is directed particularly tonovel and innovative means for the handling, processing, dewatering anddisposal of such sludge in an efficient and economical manner.

The first requirement in sludge handling is to reduce moisture, and toincrease handling ability by reducing volume. The process is usuallyreferred to as concentration, thickening, dewatering or drying,according to the amount of moisture being removed. Different sewage andwater treatment processes yield sludges of different solidsconcentrations ranging approximately from 2 to 20 percent. Furthermoisture reduction may be desirable to 25% solids for some types ofmechanical handling. Higher solids content may be desirable for specificuses or disposal methods.

Existing sludge dewatering equipment falls primarily into twocategories: (1) simple and inexpensive, but slow, sand drying beds; and(2) fast, but expensive, highly mechanized devices such as presses,vacuum filters, centrifuges, heat dryers and incinerators.

Because of the increase in the separation of both free and bound waterfrom the sludge solids effected by the instant invention, a 200% savingsis realized in the area of land required for the dewatering process, andfurther savings in the operating and maintenance costs of sludgeprocessing and utilization are experienced. No other sludge dewateringsystem is more cost effective and efficient in continued operation.

Sand drying beds require up to seven weeks to achieve cakeconcentrations of 25 to 40 percent. The slowness of this method requiresbed areas ranging from 1 to 3 square feet per capita of populationserved, depending on sludge quality and climate conditions. Theextensive land requirements of such systems have commonly forced plantsserving more than 30,000 persons into mechanical equipment. Sand bedsare however, used by 38 percent of cities serving populations over100,000.

The disadvantages of mechanical equipment stem from its high initialcost, high maintenance and operational costs, frequent breakdownproblems and large energy consumption.

SUMMARY OF THE INVENTION

Among the objects of the sludge reduction system herein described isprovision of a new and improved inexpensive system of sludge dewatering,drying, and disposal, without a high degree of mechanization, thusattaining fast, efficient drying with low land requirements, andmaintaining low initial cost and maintenance simplicity.

Another object of the invention is provision of a new and improvedsystem for sludge reduction of the character hereinabove described inwhich use is made of a plurality of adjacent filter beds utilizing castfilter plate or monolithic pour structure on a pervious sub-structurefor supporting the plate and providing drainage for collection of thefiltrate, the assemblage being of such strength and rigidity as toprovide for passage thereover of free moving mechanized sludge handlingequipment units such as front end loaders.

Another object of the invention is provision of a new and improvedsystem making use of a precast retention filter plate for use in sludgereduction or dewatering beds and the like that is comprised of granularaggregate material such as silica sand, anthracite, or aluminum oxideand bonded by epoxy resin, the sand or other media being of such sizeand in such proportion with respect to the epoxy bonding agent as toprovide optimum porosity for a high volume percolation rate suitable forsludge reduction, while at the same time exhibiting high tensile andcompressive strength sufficient to support mechanical equipment andbeing substantially chemically inert.

Still another object of the invention is provision of a new and improvedsystem for rapid sludge dewatering of the above nature including asimultaneous heating and moving device for the sludge being dewatered,and provision of a high vacuum which is applied in the void area belowthe rigid porous filter plate, thereby substantially enhancing the rateand degree of sludge dewatering or drying.

Another object of the invention is provision of a new and improvedsystem, method and means for rapid sludge dewatering of the characterdescribed including solar heating of the sludge for accelerated dryingand air pollution control. Further included among the objects is meansfor and processes of backwashing, vacuum drying, chemical treatment andchlorination of the sludge in the moisture reduction process.

A further object is provision in the basic design of the sludgedewatering system for disinfecting the sludge cake and the liquidfiltrate by chemical or irradiative means.

It should be understood that this disclosure emphasizes certain specificembodiments of the invention method, system and apparatus, and that allmodifications or alternatives equivalent thereto are within the spiritor scope of the invention as set forth in the appended claims.

DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of a sludge reduction or dewatering bedembodying the invention;

FIG. 2 is a top plan view;

FIG. 3 is a longitudinal cross-sectional view, taken along the line 3--3of FIG. 2 in the direction of the arrows;

FIG. 4 is a transverse cross-sectional view taken along the line 4--4 ofFIG. 2 in the direction of the arrows;

FIG. 4a is a view in detail of the emersion heater steam supplyassembly;

FIG. 5 is a perspective view similar to that of FIG. 1 but furtherillustrating the use of a transparent canopy for keeping out rain, highhumidity, snow etc., and for capturing solar heat and retaining it;

FIG. 6 is a longitudinal cross-sectional view similar to that of FIG. 3but illustrating an alternative support and drainage structure for themodular unit filter plates;

FIG. 7 is a schematic diagram of the sludge reduction systemillustrating its interrelation with a typical sewage treatment plant;

FIG. 8 is a side perspective view of a representative pair of sludgereduction beds equipped for electroosmosis;

FIG. 9 is an elevational view of the control panel;

FIG. 10 is a cross-sectional view of one of the beds of FIG. 8;

FIG. 11 is an enlarged sectional view on the circular line 11 of FIG.10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described below is an embodiment of the inventive apparatus chosen forpurposes of illustration as the environment for the practice of theinventive method.

There is a typical 2-unit sludge reduction bed 10. A first unit 11 isshown in its entirety and a second unit is only partially illustrated.As illustrated in FIGS. 1 through 4, each bed unit 11 comprises ashallow open-topped, box-like sludge container for receiving sludge tobe chemically pretreated and processed. The sludge container isintegrally formed of monolithically poured, reinforced concrete, oralternatively, it could be fabricated of other structural materials suchas fiberglass, concrete blocks, steel, etc. The container comprises abottom slab portion 12, upstanding sidewall portions 13 and 14, andupstanding front and back wall portions 15 and 16, respectively. Theinternal dimensions of each sludge container unit 11 may, for example,be 20×40 feet to provide in one form of the invention for the closefitting assembly therein of a plurality of modular filter plates 17which may conveniently be 2 ft.×4 ft. in size, for example. The sludgecontainers are made impervious by float finishing or by the use ofconcrete additives, of a suitable sealer, thereby preventing pollutionof the subsoil by leakage of the filtrate.

The plates 17 or monolithic poured slab serves as the filtering media inthe sludge dewatering or reduction process, and is sufficiently strongto withstand pressures imposed by mechanized dried sludge handlingequipment which will be driven over the bed from time to time for theremoval of the sludge cake at the end of the rapid sludge dewateringprocess.

The above described rigid filter plate is reinforced with inert glassfibers providing the required strength or durability during dailymechanical unloading. The filter plate is preferably installed over amass 60 of large size clean aggregate (3/4" to 11/2") which is keyedtogether by a vibratory compactor during construction. After keying, theaggregate is given a light coating of sprayed-on epoxy to preventmovement of the aggregate during daily unloading of the dewatered anddried sludge cake by mechanical means. Above the aggregate 60 is aleveling layer 60' of 6-10 sand, also keyed and sprayed with epoxy. Theaggregate and leveling layer thus installed to a depth of 12" to 24"inside the outer impervious shell provides a void area of approximately40% where the vacuum is uniformly applied over the whole poured filtermedia underdrive. This void area also provides a reservoir for the waterfiltrate during the filling-up operation. Should a power failure occur,the same area serves to collect all the filtrate.

The bottom slab portion 12 of the sludge container unit 11 in thealternative may be integrally formed with a plurality ofequidistantly-spaced, parallel longitudinally-extended ribs 18, saidribs being approximately square in cross-section and having uppersurface portions lying in the same horizontal plane. The ribs 18 are sospaced as to support marginal longitudinal underside portions ofadjacent rows of the filter plates 17.

As illustrated in FIGS. 3 and 4 the upstanding sidewall portions 15 and16 are each formed with inwardly directed ledges 19 defining shouldersat the same height as that of the ribs 18 for the support of outermarginal edge portions, at the underside of the peripherally positionedfilter plates 17. Thus, as illustrated in FIG. 4, each of thelongitudinally-extending fluid and gas flow chambers serve as a throughpassage for sludge filtrate, for backwash, or for vacuum voiding, as ishereinafter more particularly described.

As further illustrated in FIGS. 3 and 4, the upper surface of the bottomslab portion 12 of the sludge container 11 is sloped downwardly fromback to front so as to provide for gravity drainage of sludge filtrateor processing fluids into a transversely-extending manifold 22 formedwithin the sludge container.

Drainage of the sludge container 11 is provided for by a manifold fluidconduit 23 communicating with the bottom of the manifold 22 andextending outwardly of the upstanding sidewall 13. There is a gasconduit 24 also extending through the sidewall 13 at the top of themanifold. The manifold and conduits 23 and 24 extend through individualelectric control valves 25 and 26 to join at a common inlet conduit 27leading to a sump pump 28. The output of the sump pump 28 is fed throughconduits 29 and 30 to sludge filtrate return line 31 and chemicalsolution return line 32, respectively, as controlled by respectivecontrol valves 33 and 34, as shown in FIGS. 1 and 2. As many as four ormore rapid sludge dewatering units, if desired, may be connected in abasic design to one central vacuum reservoir and effluent tank by meansof cross-connecting lines and control valves, thereby conservingmaterial and capital investment.

Means is provided for filling one or the other or both of theside-by-side sludge container units 11, 11a, comprising the two-unitsludge reduction bed 10, with fluid waste sludge. To this end, adjacentzones of the backwall portion 16 of the side-by-side sludge containers11, 11a are joined by a rearwardly-extending sludge-receiving fillingand mixing box 39. Slide gates 40 and 40a in respective backwallportions 16 and 16a permit selective passage of liquid sludge fed intothe filling and mixing box 39 into one or the other or both of thesludge containers 11 and 11a selectively, depending upon the volume ofsludge being fed into the beds for treatment. A comparatively largediameter conduit 41 discharges through a manually controlled valve 42into the filling and mixing box 39, said sludge being pumped from atypical sewage or water treatment facility for disposal.

Although not shown in the drawings, an important factor in the practiceof the method herein disclosed is a conventional mobile mechanizedvehicle capable of entering on the filter plate, there to be loaded withsludge cake for removal. Such mobile units are commercially availableand if not already equipped with a scraper and lifting mechanism,accessories of such description can be installed.

Mobile mechanized units most advantageous for use in the system heredisclosed are four wheel vehicles of sufficient length and breadth tocarry the operator and an appreciable load of sludge cake. With wheelsproviding traction, the operator can mount and drive to and from aloading location on the filter plate. It is of consequence, therefore,that the length and breadth of each sludge container 11, 11a be as largeas described to allow the mobile unit to move about while the sludgecake is being scraped clear from the top surface of the filter plate,and ultimately loaded on the mobile unit for transportation elsewhere.

The nature of the mechanized units is clearly such that care must betaken in the construction of the filter plate and its support to carrythe load of the mobile mechanized unit and facilitate repeated cleaningand scraping cycles for removal of sludge cake when dried, whileavoiding damage to the filter plate's upper surface.

Clearly, because of the semifluid character of the sludge, side, frontand back wall portions 13, 14, 15 and 16 must provide a liquid-tightcontainer while the dewatering of the sludge is taking place. Toaccommodate entrance and exit of the mobile mechanized units, a slidegate, such as gate 43, is provided in the back wall portion 16 of thesludge container unit 11. A similar gate (not shown) is provided for theback wall 16a of the sludge container unit 11a and for each of anyadditional sludge container units which may make up an installation.Accordingly the gate must have a liquid tight fit when closed, andprovide an opening wide enough to accommodate a loaded mobile mechanizedunit such, for example, as a front end loader.

Optional means is provided for simultaneously heating and moving sludgein the sludge container units 11 and 11a during the dewatering process.This system will provide for a 7% solid sludge cake content therebyfacilitating prolongation of the drying process to achieve a driercondition. In other words, by closing the cracks in the sludge as itstarts to crack at 7%, the sludge heating and moving means serve toextend the dewatering process, thus producing drier sludge cake in ashorter time.

More specifically, there is a transverse support bar 44 which straddlesthe upstanding sidewall portions 13 and 14 of each of the sludgecontainer units 11 and 11a. The ends of the support bar 44 carry rollerbearings 45 which ride along the upper surfaces of the sludge containersidewall portions 13 and 14 to minimize frictional resistance in themovement of said support bar reciprocally between the front and backwallportions 15 and 16, respectively, of the associated sludge containerunit.

A continuous drive cable 46 is looped between an idler pulley 47upstanding from and fixed with respect to the center of the backwallportion 16 and the drive pulley 48 of a reversible electric motor 49.The electric motor is fixed with respect to and extends upwardly from acentral portion of the container front wall portion 15.

The lower loop portion or run of the drive cable 46 extends through andis attached to a central portion of the transverse support bar 44, asindicated at 50 in FIG. 3. The transverse support bar 44 carries aplurality, three in the embodiment illustrated, oftransversely-extending emersion heater conduits 51a, 51b and 51c. Theconduits are positioned at various heights to extend at various levelsinto the semifluid mass of sludge fed into a sludge container 11 fortreatment.

As illustrated schematically in FIG. 4a, a steam generator 52 suppliessuperheated steam through the emersion heater conduits 51a, 51b, and51c, connected in series, through flexible conduits 53 and 54. Anelectric energization circuit (not shown) for the reversible drive motor49 serves to reciprocally move the transverse support bar 44 and itsassociated emersion heater conduits back and forth within the sludgecontainer units 11 and 11a during the sludge dewatering or dryingprocess. Alternatively, elongated electric heating units could be used.

The sludge moving and heating means, comprising the reciprocativetransverse support bar 44 and its associated emersion heater conduits51a, 51b and 51c, serves primarily as a device for uniformly moving thesludge to minimize any tendency to the formation of shrinkage crackstherein, particularly during the vacuum drying process. The sludge,being a gelatinous mass, is much more easily cut for mixing by theheated conduits as compared with unheated conduits, to which the sludgewould have tendency to stick. The heat imparted to the sludge beingmixed also renders it more fluid, particularly in cold weather, therebyfurther enhancing the time efficiency of the vacuum drying process.

An important feature of the invention resides in the composition of thefilter plates 17. They must not only exhibit the requisite porosity forrapid and efficient filtering, be inert with respect to reaction withany of the various caustic and corrosive chemicals commonly found insludge and sludge treatment, but must also withstand loads imposed bythe mobile mechanical units employed to remove the dried sludge cake. Ithas been established that a 3" thick filter plate having the requiredstrength, resistance to chemicals and filtering or percolation rate isobtained with a mixture of angular silica filter sand having a sievesize of between 6 and 10 millimeters used as the aggregate, and abonding agent of chatahoochie epoxy in the ratio of one pound of theepoxy to 20 pounds of the aggregate.

An effective filter plate is shown in FIG. 11 of the drawings. The plate17, whether individual plates or a poured slab, is 11/2 to 2 inchesthick and has a lower permeable layer 17a of 3/4 inch to 11/2 inches ofaggregate. This is rigidified, meaning that the particles are workedinto a mass such that they are substantially all interlocked with eachother, to inhibit any further shifting or movement. The choice ofaggregate is such, however, that the layers remains permeable and permitfree flow of water withdrawn during the dewatering cycle.

Above the lower permeable layer is a leveling layer 17b of about 1/4"thickness for which 6-20 sand is suitable. This leveling layer 17b isalso rigidified by working or tamping similar to, and for the samepurpose as, the lower permeable layer, leaving it firm, and at the sametime, permeable.

Above the leveling layer is a layer 17c of sharp, fine 16 gritanthracite and/or aluminum oxide aggregate also rigidified thoughpermeable. Individual sharp points of the 16 grit anthracite and/oraluminum oxide present a "sand paper" like character for the uppersurface of the plate 17. Included in all the layers is an appropriatebinder such as epoxy, which, after hardening, renders the multi-layeredfilter plate monolithic and physically strong to be heavy equipmenttraffic rated as described.

The columnar structure 60, 60', whether gravel as shown in FIG. 11, orribs, resting on the impermeable bottom slab portion 12, substantiallyas has been already described, leveled off by the layer 60' of 6-10sand, supports the filter plate 17 at a location within wall portions13, 14, 15 and 16.

In a second form of the invention shown in FIG. 5, a transparent canopy55 extends over the sludge container 11, fully enclosing the same,thereby serving not only as a rain water shed, but also providing forcapturing solar heat to accelerate the sludge drying process. The canopymay also serve to contain heat generated by fuel-fired heaters.Preferably, the canopy may be of a dark colored vinyl sheet of the typecommonly used in hothouses to create the so-called greenhouse effect.The canopy also serves to contain gaseous pollutants, often malodorous,which might otherwise contaminate the surrounding atmosphere. Thesegaseous pollutants can be recycled to the primary treatment facilityfrom which the sludge is obtained for sludge reduction, to be reabsorbedin the fluid state. Solar heat collector panels provide warm dry air tobe pulled through while the vacuum below is applying an acceleratingdewatering assist.

By heating air in the space above the sludge bed, whether by solarenergy in the event a canopy is employed, or by some other appropriateheater, multiple benefits result. Warm air when drawn through the filterplate with the filtrate appreciably accelerates drying of the sludgecake.

The drawing of warm air warms the semi-liquid sludge and acclerates thedewatering process, and, consequently, the drying of residue.

Occasionally reversing air flow through the filter plate has addedbenefits, namely loosening up the accumulation of partially dewateredsludge, as well as enhancing the dewatering cycle.

FIG. 6 is a longitudinal cross-sectional view, similar to that of FIG. 3but with portions broken away, illustrating a simplified form of sludgecontainer embodying the invention. The structure there shown is oneparticularly well-suited for installation on land area having highbearing strength such as coral rock or naturally dense sand or sand androck mixtures. Under such conditions, as illustrated in FIG. 6, theimpervious concrete bottom slab can be eliminated and the upstandingsidewalls 13a and front and backwall portions 15a and 16a can beintegrally formed of reinforced poured concrete with a peripheralfooting 56.

As further illustrated in FIG. 6 and as indicated at 57, the interiorbottom surface will be sloped for drainage from back to front into arecessed, transversely-extending manifold 58 corresponding with themanifold 22 of the embodiment illustrated in FIGS. 1 through 4. Amoisture impervious layer 59 of 12 gauge Neoprene sheeting may cover theinterior of the sludge container thus formed. The two to four inch thicklayer of coarse clean gravel 60 on the bottom area of the Neoprenesheeting 59 presents a horizontal upper bearing surface upon which thefilter blocks 17 are placed, as in the embodiment of the inventionillustrated in FIGS. 1 through 4. Preferably, the gravel will be of asieve size from 3/4 to 1 inch.

The spaces within the mass of gravel form a continuous flow passage forfiltrate which passes through the filter plate. Accordingly the gravelserves a double purpose, namely, that of establishing flow passages andthat of acting as a columar support for the filter plates throughout thearea of the sludge container unit.

Instead of using modular plates 17, the filtering medium can be a singlemonolithic slab (not shown) of the silica sand and epoxy binding agentmixture. Gravel is effective for use as a combined support and structureforming flow passage. The monolithic slab may be supported by 3/4" to11/2" aggregate, providing for the approximately 40% void area, or byribs like the ribs 18 of FIGS. 1 and 4. The use and operation of theinvention illustrated in FIG. 6 is otherwise the same as the principalembodiment of FIGS. 1 through 4.

FIG. 7 illustrates, schematically, a typical sewage treatment plant andassociated two-unit sludge reduction system embodying the invention. Thesewage treatment system may comprise, for example, the usual raw sewageinput surge tank 61 feeding successive aeration, settling and chlorinecontact tanks 62, 63 and 64, respectively, which serve to reduce thesolid matter to a substantially homogeneous sludge discharged to asludge holding tank 65.

Reagent for ion removal is supplied to the aeration tank 62 from asupply tank 66 through a pump 67. Effluent is passed from the chlorinecontact tank 64 to a tertiary filter 68 for further removal of suspendedsolids, and the filtered effluent is discharged through conduit 69 to adrain field or other suitable means of disposal. The filtered solids arereturned through a conduit 70 to the surge tank 61 for reprocessing andeventual disposable in the sludge.

Supply water is fed through conduit 71 and control valve 72 into achlorinator 73 discharging chlorine in solution into the chlorinecontact tank 64 for treatment of the effluent therein. The water supplyline 71 also serves to feed chemicals in solution to the sludgecontainer units 11, 11a in the sludge treatment process.

In operation of the sludge dewatering or reduction system in theembodiment illustrated in FIGS. 1 through 4 and 7, fluid sludge to betreated is pumped through a sludge pump 74, conduit 41 and control valve42 into the common sludge filling and mixing box 39, wherein it will mixto a more or less homogeneous mass, then allowed to flow into one or theother or both of the sludge container units 11 and 11a by appropriatelyopening the sludge gates 40 and 40a respectively. The sludge containerunits 11 and 11a will be filled to a depth of approximately 12 inches,whereupon the liquid fraction of sludge will begin to percolate throughthe filter plates 17 and drain into the manifold 22 at the front of theunit. This separated filtrate will ordinarily be returned by the sumppumps 28 to the surge tank 61 through appropriately valved conduits 75for retreatment (See FIG. 7).

Various forms of treatment of the sludge in the sludge reduction unitsis provided for, depending upon the quality of the sludge to be treated.For example, washing with clean water or elutriation may be required ofsludges high in soluble inhibitors of coagulation. To this end, supplywater fed through conduit 71 and passing through chemical feed tank 76is pumped into backwash chemical feed line 35 by backwash pump 77.Sludges rich in greases may require lime conditioning. Industrial wastesludges may require other chemical conditionings which can be suppliedas a backwash solution by adding appropriate chemicals to the chemicalfeed tank 76. Alternatively, chemical treatment solutions can be addeddirectly to the sludge by passing supply water solvent through achemical feed tank 78 and pump 79 to conduits 80 and 80a discharginginto the sludge container units 11 and 11a respectively.

It is also to be understood that cracking of the sludge cake duringvacuum drying can also be inhibited by the use of chemical coagulantsadded to the sludge at the filling pump.

In following a cyclical pattern for adjacent sludge beds the processincludes pretreating raw sewage with a polymer coagulant to break upsolids and flocculate the sludge particles. Numerous tests dictate thatthe sludge be treated to create large soft particles and soft particlesof various molecular and sieve sizes. The larger particles gain weightand then being heavy are settled first. Smaller finer particles aresettled thereafter and are trapped by the already settled largerparticles. Settling of the particles is assisted by application of avacuum beneath the filter plate.

The added coagulant, by flocculating the sludge, causes formation ofsoft particles or masses of various sizes. As stated, the largerparticles tend to settle first by gravity action and spread a gel-likeslime layer over the top of the filter plate. Free water drains rapidlythrough the relatively large porous spaces of the initially formedlayer, and through the filter plate. This initially formed layer at thesame time traps the smaller particles which continue to settle out.

When a vacuum is applied as a next step, a fast liquid filtrate flow isinduced through the porous sludge cake and the filter plate, rapidlydewatering the sludge.

The choice of anthracite and/or aluminum oxide for the top layer of themonolithic filter plate is of significance. Anthracite and/or aluminumoxide provides sharp, upwardly extending points and these tend to piercethe material of the larger particles in the slime layer adding to thespeed and completeness of the dewatering.

A circumstance of some note is that a vacuum applied quickly afterformation of the flocculated slime expedites formation of the cake. Thisis directly opposite to the cake formation in a standard vacuum filter,in which the smaller particles are picked up first in the cake or, atbest, a homogeneous sludge cake is achieved thus forming a sludge cakewhich is more permeable and harder to dewater. Wide latitude is possiblein strength of the vacuum applied which may vary with circumstancesbetween 1 and 27 inches. The sludge cake will continue to dry whilewater is pulled from the floc. This differential type of cake formationproduces a thicker cake and delays formation of cracks, permitting themaintenance of the vacuum for a longer period. In many cases, this is asfar as the dewatering need go for quick and economical disposal since 75to 80% of the water will have been removed. A final optional stage canbe the further dewatering of the sludge cake by drying the adherentwater by hot air pulled by a vacuum with or without mechanical raking.

Pneumatic lines 36 extend from the sludge container sumps to areversible pneumatic system 81 which, when operated in its vacuum mode,provides a substantially reduced pressure beneath the filter plates 17,thereby greatly accelerating the dewatering process. An added benefit ofapplied vacuum will be the suctioning for inplant disposal of anyundesirable gases and odors. This vacuum drawn filtrate will normally bereturned to the sewage treatment facility surge tank 61 for reprocessingalong with the gravity filtered or percolated filtrate.

In FIG. 8 is shown a pair of sludge containers 11' connected in tandemutilizing common sidewall 13'. On other respects the containers areidentical, each comprising, as shown, a second side wall 14' with frontwall and back wall portions 15' and 16', respectively, surrounding animpervious bottom slab 12'. The customary monolithic filter plate 17 ishere shown extending over the entire upper surface of a level,rigidified supporting course 60 of aggregate.

Of special consequence in this form of the invention is the presence ofan anode 85 and cathode 86. Where the side walls of the container 11'are to be about 12" high above the filter plate the anode is fastened tothe wall structure about 4" above the top level of the filter plate. Theanode is preferably copper and located so as to be continuously immersedin the semi-liquid sludge.

This anode extends around all four side walls.

The cathode 86, likewise extending around all four side walls, islocated at the bottom of the body of semi-liquid sludge, preferably atand touching the monolithic filter plate itself. The material of thecathode is non-ferrous and a material other than copper. Carbon is foundto be satisfactory. An appropriate non-ferrous metal may be, forexample, zinc or aluminum.

The anode and cathode are not cconnected to any source of electric powerbut serve, instead, as anode and cathode in a bath of electrolite, asfor example, the semi-liquid sludge. In this arrangement the electrodesestablish an electroosmosis of the sludge causing the solid sludgeparticles to flow to the cathode and the water molecules to flow to theanode. The flow of water thus induced accelerates the speed ofdewatering and, in fact, significantly contributes to the increasedspeed and completeness of dewatering and formation of sludge cake.Details of construction of the electrodes have not been included in thedisclosure inasmuch as such details may vary considerably withoutdeparting from the scope of the invention.

For convenience there is shown a control panel with appropriateinstrumentation 87 which has controls to motivate the sundry mechanicalmeans such as the coagulant metering pump 88, blower control 89, valvesfor the various lines and the sump pump and vacuum chambers 91.

Following dewatering, isotope irradiation or addition of a chlorinesolution may be employed to disinfect and deodorize the reduced sludge.This can be effected by supplying chlorine to the chemical feed tanks76, 78 for backwash or forward wash. An appropriately valved chlorinesolution return line 82 provides for recycling the chlorine solutionthough the sludge as pumped by sump pumps 28 for efficient use of thechlorine. It will be understood that the various above describedfiltration, backwash and chemical treatment steps, both liquid andgaseous, in the sludge reduction process can be programmed for automaticoperation, with selective variations depending upon the quality of thesludge to be dewatered or reduced.

Among the advantages of the sludge reduction dewatering system embodyingthe invention are the following:

1. Sludge having a dry solids content of 2% can be dewatered inapproximately five hours sufficiently to be removed from the sludgecontainers mechanically and transferred by open truck to a disposal sitewithout difficulty or loss.

2. Sludge can be dewatered to 15% dry solids in approximately eighthours.

3. Recapture of approximately 99.5% of the solids in the sludgereduction process.

4. Chemical disinfecting of the dry sludge is easily effected.

5. Disinfection may be obtained by isotope irradiation.

6. At least 40 times less land area is required as compared with sludgedewatering systems heretofore employed.

7. Substantial reduction in odor pollution of the surroundingatmosphere.

I hereby claim as my invention:
 1. An improved process for dewateringsludge on a filter medium, which sludge has been pretreated, whererequired, with a chemical conditioner to form soft masses of varioussizes including larger and smaller sludge masses, wherein theimprovement comprises:(a) passing the sludge onto the upper surface of amulti-layered filter plate comprising:(i) a thin top layer having asmooth, hard upper surface and a lower surface and consistingessentially of fine aluminum oxide particles, rigidified and then bondedtogether with a binding agent which does not impede high volumepercolation; and (ii) a bottom support layer composed of relativelycoarse aggregate particles, rigidified and then bonded together and tothe lower surface of the top layer with a binding agent which does notimpede high volume percolation, which filter plate is monolithic andstructurally rugged enough to support sludge removal means on the uppersurface of the top layer of the filter plate without damage to thefilter plate; (b) permitting the larger sludge masses to settle by forceof gravity so that they are pierced by the aluminum oxide particles onthe upper surface of the filter plate and form a layer over the uppersurface of the filter plate; (c) applying a vacuum of sufficientstrength to draw filtrate from the sludge through the layer of largersludge masses on the upper surface of the filter plate and through thefilter plate, but not so strong as to break down the larger sludgemasses, such that the smaller sludge masses are trapped by the alreadysettled larger sludge masses, until the sludge is dewatered to an extendmaking it removable by sludge removal means; and (d) removing thedewatered sludge from the surface of the filter plate by sludge removalmeans;wherein the process is characterized by rapid dewatering andsubstantial self-filtering action.
 2. The process of claim 1 wherein thestrength of the vacuum is between 1 and 27 inches of mercury.
 3. Theprocess of claim 2 wherein the strength of the vacuum is between 10 and15 inches of mercury.
 4. The process of claim 1 wherein the sludge isdewatered to a solids content of 5% or greater.
 5. The process of claim1 wherein a sludge having an initial solids content of up to 8% isdewatered within approximately 50 hours sufficiently such that it isremovable by sludge removal means.
 6. The process of claim 1 wherein thesludge is dewatered to a solids content of about 15% withinapproximately 8 hours.
 7. The process of claim 1 wherein the sludge isprewashed with clean water to remove soluble inhibitors of coagulation.8. The process of claim 1 wherein the sludge is pretreated with lime. 9.The process of claim 1 wherein the process is carried out in cyclicalfashion on a plurality of adjacent filter beds.
 10. The process of claim1 wherein the sludge removal means comprises one or more front endloaders.
 11. The process of claim 1 wherein the sludge is heated andagitated by heating and agitating means during dewatering.
 12. Theprocess of claim 1 wherein the filter plate is housed within atransparent canopy.
 13. The process of claim 1 wherein the dewatering isaccelerated through electroosmosis.
 14. The process of claim 1 furthercomprising disinfecting the reduced sludge after it is dewatered. 15.The process of claim 1 wherein the thickness of the top layer of thefilter plate is between about 1/4 and about 1 inch.
 16. The process ofclaim 1 wherein the thickness of the bottom support layer of the filterplate is between about 11/2 and about 2 inches.
 17. The process of claim1 wherein the binding agent of the filter plate is epoxy.
 18. Theprocess of claim 1 wherein the aluminum oxide particles of the filterplate are 16 grit alumina.
 19. The process of claim 1 wherein thethickness of the top layer of the filter plate is between about andabout 1/4 and about 1 inch, and the thickness of the bottom supportlayer is between about 3/4 and about 11/2 inches.
 20. The process ofclaim 1 wherein the plate is reinforced.
 21. A multi-layered filterplate for dewatering sludge comprising an upper filter layer and a lowersupport layer,a. the upper filter layer comprising alumina particles anda means for rigidly bonding the particles together to define the upperfilter layer, the particles being of a size that does not impede highvolume percolation, and the bonding means being of a type that does notimpede high volume percolation; b. the lower support layer comprisingaggregate material and a means for rigidly bonding the aggregatematerial together to define the lower support layer, the aggregatematerial being of a size that is greater than the size of the particlesin the upper filter layer and that does not impede high volumepercolation, and the bonding means being of a type that does not impedehigh volume percolation;the filter plate being of a monolithicconstruction and being of sufficient structural strength to supportmobile mechanized sludge removal means.
 22. A filter plate as in claim21 wherein the thickness of the top layer is between about 1/4 and about1 inch.
 23. A filter plate as in claim 22 wherein the thickness of thebottom support layer is between about 3/4 and about 11/2 inches.
 24. Afilter plate as in claim 22 wherein the binding agent is epoxy.
 25. Afilter plate as in claim 21 wherein the thickness of the top layer isbetween about 1/4 and about 1 inch, and the thickness of the bottomsupport layer is between about 3/4 and about 11/2 inches.
 26. A filterplate as in claim 21 wherein the plate is reinforced.
 27. In a systemfor removing a substantial amount of water from water containing sludge,the system being of the type that includes a rigid tank having an upperzone for receiving the water containing sludge and for collecting sludgefollowing the substantial removal of the water, having a lower zone forcollecting the water that has been removed from the sludge, and havingan intermediate filter assembly interposed between the upper and lowerzones, an improved filter assembly comprising an upper filter layer anda lower support layer, the filter assembly being of a monolithicconstruction and being of sufficient structural strength to supportmobile mechanized sludge removal means, the upper filter layercomprising alumina particles and a means for rigidly bonding theparticles together to define the upper filter layer, the particles beingof a size that does not impede high volume percolation, and the bondingmeans being of a type that does not impede high volume percolation, thelower filter layer comprising aggregate material and a means for bondingthe aggregate material together to define the lower support layer, theaggregate material being of a size that is greater than the size of theparticles in the upper filter layer and that does not impede high volumepercolation, and the bonding means being of a type that does not impedehigh volume percolation.